There are many applications of bladed rotor assemblies for generating pressure and creating flow in air handling systems. One example of such an application is a gas turbine engine.
A conventional gas turbine engine includes compressor and turbine stages, each of which is essentially an air handling system. When incorporated as part of a turbofan engine the initial compressor stage, or fan, becomes increasingly important to the overall efficiency of the engine.
In order to maximise the operating efficiency of the fan system it is desirable to minimise the clearance between the radial ends of the fan blades and the internal surface of the fan casing.
However, although setting a very small clearance will increase the efficiency of the fan system, such a small clearance can increase the probability of contact between the fan blade tip and the casing. This contact between fan blade tip and casing may occur either during transient conditions (i.e. gust loading) or during steady state operation, for example due to thermal growth resulting from extended high temperature operation.
Consequently, in conventional fan systems it is known to either set a minimum blade tip clearance that will ensure no blade tip to casing contact degrading initial performance but minimising further degradation through life, or alternatively to accommodate such contact by including an abradable lining portion within the fan casing resulting in good initial performance but relatively worse performance at high life. The effect on engine efficiency of these contrasting approaches is illustrated in FIG. 1.
In systems employing an abradable liner, the small blade tip clearance will initially provide good performance. However, the accumulation of rubbing damage to the abradable liner will produce radial steps corresponding to the leading and trailing edges of the fan blades. These steps will disrupt the airflow through the fan and reduce the efficiency of the fan system.
It is therefore desirable to be able to maximise the efficiency of the fan system without introducing radial steps in the abradable liner, which will disrupt the airflow through the fan system and reduce its efficiency.